PURPOSE: To reduce the residual output (heat) at low cost by continuously circulating secondary sodium within a secondary cooling system, and a carrying cooled air from the bottom part to the upper part in the annular space between an outer wall and a steam generator.

CONSTITUTION: In order to remove the residual output even after a reactor is stopped, secondary liquid sodium is continuously circulated within a secondary cooling system 1 by a pump 10. Thus, the secondary sodium is circulated within an intermediate heat exchanger 6 to cool the primary sodium absorbing the residual energy of a reactor core 5. The secondary sodium recovered from the outlet of the intermediate heat exchanger 6 through a discharge pipe 9 is returned to the upper part of a steam generator 15, circulated within a casing 15a, and discharged out of the casing 15a through a pipe 12. An annular space and a fin are provided between an outer wall and the casing 15a. Cooled air is carried from the bottom part of the annular space to the upper part, guided to the casing 15a by the fin to remove the residual heat propagated from the secondary sodium without using an expensive additional device.